Objective(s): Palliation of hypoplastic left heart syndrome remains challenging. Further delineation of the incidence and nature of pathology in this population can aid in both prognosis and furthering care for these patients.Methods: We performed a pathological review of 117 hypoplastic left heart syndrome specimens among the anatomic collections of two institutions. Assessment of atrioventricular valves, atrial appendage, pulmonary veins, and left ventricular morphology was performed.Results: Among the 117 specimens, 37 were mitral atresia/aortic atresia (MA/AA), 24 were mitral stenosis/aortic stenosis (MS/AS), and 56 were mitral stenosis/aortic atresia (MS/AA). Sixteen specimens (14%) had undergone various stages of single ventricle palliation. Fifty-seven patients (49%) had tricuspid valve dysplasia. Left atrial appendage hypoplasia was evident in 33 patients, and this was higher in the MA/AA group (51%) compared to MS/AS (17%) or MS/AA (18%, p=0.001). Pulmonary vein hypertrophy was more common in the MA/AA group (22%) than the other groups (8% MS/AS, 7% MS/AA, p=0.047). A slit-like left ventricle was much more prevalent in the MA/AA specimens (92%), compared to MS/AS (19%) or MS/AA (2%, p<0.001), while globular left ventricle was more prevalent in MS/AA (96%) and MS/AS (62%). Well-developed but proportionally undersized left ventricles were evident in 4 MS/AS specimens (19%) and one MS/AA specimen (2%). LV EFE was substantially more common in the MS/AA specimens (77%) and MS/AS specimens (75%) compared to the MA/AA specimens (8%, p<0.001).Conclusions: Overall pathologic abnormalities were more common in the MA/AA group and almost half had dysplastic tricuspid valves.